1. Field of the Invention
The invention relates to antifoam formulations comprising (A) antifoam agents based on siloxanes and (B) branched polyether/polysiloxane copolymers, and the use thereof for the defoaming of aqueous media, in particular media obtained in pulp production.
2. Background Art
In many liquids which contain surface-active compounds as desired or undesired components, particularly aqueous systems, problems may arise due to foam formation if these systems are brought into more or less intensive contact with gaseous substances, for example during the gassing of wastewaters, during the intensive stirring of liquids, in distillation, scrubbing or dyeing processes or in filling processes. This foam can be controlled by a mechanical method or by the addition of antifoams. Siloxane-based antifoams have proven particularly useful.
Antifoams based on siloxanes are prepared, for example according to DD-A 056 762, by heating hydrophilic silica in polydimethylsiloxanes. The process is very complicated, but the efficiency of the antifoams thus prepared is still not satisfactory. The distribution of hydrophobic silica in a polydimethylsiloxane, for example according to DE-A 29 25 722, is a more rational process, but the efficiency of the antifoams obtained is likewise worthy of improvement.
The use of modified polyorganosiloxanes in antifoam preparations is also known. Thus, for example, the use of branched polysiloxane antifoams in combination with polyether/polysiloxane copolymers as antifoams is recommended for pulp production (EP-A 341 952). Polyether/polysiloxane copolymers are said to have a positive effect also in combination with mineral oils as a carrier oil (U.S. Pat. No. 5,523,019). According to WO 98/000216, siloxanes having dimethyl-3-hydroxypropylpolyoxyethylenepolyoxypropylene groups are said to be particularly suitable as surfactants in antifoam formulations.
EP-A 663 225 and EP-A 1076073 describe, respectively, crosslinked and branched polyorganosiloxanes which carry at least one polyether group, as one of the two components of an antifoam formulation. The crosslinking is effected via alkylene groups, via polydimethylsiloxanes or via polyether groups. As a result of the linkage via Si—C bonds, the products are stable to hydrolysis. In contrast, the process proposed in U.S. Pat. No. 5,625,024 leads to linkages via Si—O—C groups, which are not stable to hydrolysis, particularly in acidic or basic media, and thus such compositions rapidly lose their efficiency in a foaming aqueous medium.
In addition to the drawbacks discussed above, the antifoam formulations prepared according to the prior art do not always have sufficient efficiency.